The present invention generally relates to a power conversion apparatus. More specifically, the present invention relates to a power conversion apparatus having a control circuit capable of achieving soft-switching of a switching element.
In a power conversion apparatus such as a converter or an inverter, various circuits having a soft-switching function are currently under development in order to reduce a switching loss in each switching element and increase a switching frequency.
For example, U.S. Pat. No. 5,047,913 discloses a converter in which the voltage of a DC (Direct Current) power supply is split into two one-half voltages, by a pair of series-connected capacitors, and the junction between the capacitors is connected to the junction between a pair of series-connected main switches, through a circuit including a series connection of a bidirectional switch and an inductor. A load is connected to the junction between the main switches. A diode is connected in parallel with each of the main switches to allow each of the diodes to become reverse biased with respect to the DC power supply. A snubber capacitor is connected in parallel with each of the main switches. In the power conversion apparatus described in this US Patent, it is intended to obtain a specific condition for achieving soft-switching of all switches by making up an auxiliary resonant commutation circuit with the circuit of the bidirectional switch and the inductor makes up and performing resonant operation through the auxiliary resonant commutation circuit.
In the above circuit, while such soft-switching is achieved when each of the main switches are turned on, a turn-off loss is caused by turning off each of the main switches. Specifically, in this circuit, for commutating from one of the main switches to the other main switch, the bidirectional switch is first turned on in the state when a load current is refluxed to the diode connected in parallel with the one main switch, so as to generate a resonant state. Then, when the current of the inductor increases up to a sufficient extent for commutation, the above one main switch is turned off. However, the control taught in this US Patent is inevitably involved with turn-off loss of the main switches. The necessity for detecting of a resonant current required for commutation also forces a complicated control.
Furthermore, in this prior art circuit, for preventing the accumulated energy in the snubber capacitor connected to each of the main switches from being consumed as short-circuit loss in the main switches under light load, the auxiliary switch is turned on before switching the main switches to commutate from one of the main switches to the other main switch. In this control, upon turning on the auxiliary switch, the inductor current starts passing through the one main switch along with the load current. When the current goes up to a certain threshold, the one main switch is turned off to charge or discharge the energy in each of the snubber capacitors. After the completion of commutation, the other main switch is turned on. Thus, a turn-on at zero current is achieved in the other main switch, and the energy of the snubber capacitor does not become a loss. However, the control taught in this US Patent is undesirably involved with complexity in control due to the switching control according to detecting the inductor current required for commutation.
Moreover, in the control taught by the US Patent, for commutating with passing the load current through the diode connected in parallel to the other main switch in the state when the load current passes through the other main switch, the commutation in large load current is achieved based on the load current without activating the auxiliary resonant commutation circuit. In small load current, the commutation is achieved based on the sum of the resonant current and the load current with activating the auxiliary resonant commutation circuit. This control process undesirably involves a ripple voltage caused by operating the power conversion apparatus as an inverter. Specifically, when the power conversion apparatus is operated as an inverter according to this control process, the current of the auxiliary resonant commutation circuit generates a ripple having the same cycle as that of the output voltage of the inverter at the midpoint of the potential of the capacitor connected in series with the DC power supply. If it is attempted to suppress this ripple voltage within the allowable range of voltage variation, it will be required to employ capacitors having larger capacity than those of conventional circuits, resulting in larger size components.
Japan Patent Laid-Open Publication No. Hei 07-115775 discloses an inverter in which one ends of auxiliary switches is connected respectively to a first split point having a first potential and a second split point having a second potential, the other ends of the auxiliary switches being connected with each other, and the junction between the auxiliary switches being connected with the junction between a pair of main switches through a resonant inductor. A snubber capacitor is connected in parallel with each of the main switches. A diode is connected in parallel with each of the main switches and in the reverse bias direction with respect to a DC power supply. In the circuit for a power conversion apparatus disclosed in this Patent Laid-Open Publication, an auxiliary resonant commutation circuit is formed of the auxiliary switches, the resonant inductor, and the snubber capacitors each connected in parallel with the main switches so as to achieve soft-switching based on resonant current passing through the formed resonant circuit.
The power conversion apparatus described in this Patent Laid-Open Publication employs a battery to obtain the first and second potentials. This undesirably makes the circuit larger in size. If a capacitor is used to provide smaller size apparatus, a ripple voltage having the same cycle as that of the output voltage of the inverter will be generated between the first and second potentials. This causes the same problem as that of the circuit described in the above US Patent occurs.
In view of the aforementioned problem in conventional power conversion apparatuses intended for achieving soft-switching, it is therefore a primary object of the present invention to provide an improved power conversion apparatus comprising a control circuit for generating a switching signal at the timing allowing soft-switching to be achieved, and free from any occurrence of ripple.
In order to achieve this object, a power conversion apparatus according to the present invention includes at least a pair of main switches composed of serial-connected first and second main switches, wherein one of the ends of the first main switch is connected with the positive side of a DC power supply, and one of the ends of the second main switch is connected to the negative side of the DC power supply. The power conversion apparatus further includes a diode connected in parallel with each of the main switches so as to become reverse biased with respect to the DC power supply, a main-switch snubber capacitor connected in parallel with each of the main switches, a load connected with the junction between the pair of main switches, and a control circuit for forming a switching signal for controlling the switching operation of the main switches by using a load voltage and/or a load current as an input thereof, wherein the main switches are controllably switched according the switching signal from the control circuit so as to generate an output. Based on the above construction, the power conversion apparatus of the present invention comprises a first auxiliary resonant circuit including serial-connected first and second auxiliary switches and a resonant inductor connected in series with the second auxiliary switch, wherein the first auxiliary resonant circuit is connected with each of the positive side of the DC power supply and the junction between the pair of main switches. The power conversion apparatus further includes a diode connected to each of the first and second auxiliary switches so as to become reverse biased with respect to the DC power supply, and voltage detecting means for detecting the voltage across each of the main switches and auxiliary switches. The control circuit is applied with a voltage signal as an input representing the voltage across each of the main switches and auxiliary switches from the voltage detecting means, and then the control circuit provides a turn-on signal to the first and second auxiliary switches according to the input before a turn-on signal as the switching signal is provided to the first main switch. The control circuit also provides the turn-on signal to the first and second auxiliary switches when the load current passes through the diode connected in parallel with the second main switch, so as to turn on the first and second auxiliary switches to direct the current from the DC power supply to the resonant inductor. Then, a resonant circuit is formed by the resonant inductor and the snubber capacitors connected in parallel with the main switches when the current of the resonant inductor goes up approximately to the load current, and the control circuit outputs a signal for turning on the first main switch when the voltage across the first main switch goes down approximately to zero through the resonance in the resonance circuit.
In another aspect of the present invention, the power conversion apparatus may includes serial-connected third and fourth auxiliary switches which are connected between the negative side of the DC power supply and the inductor so as to form a second auxiliary resonant circuit. Further, an auxiliary-switch snubber capacitor is connected between the junction between the first and second auxiliary switches and the junction between the third and fourth auxiliary switches, and a diode is connected to each of the third and fourth auxiliary switches so as to become reverse biased with respect to the DC power supply. In this case, the control circuit provides a turn-off signal to the first auxiliary switch when the charged voltage of the auxiliary-switch snubber capacitor is approximately equal to the voltage of the DC power supply after the first main switch is turned on, and to provide the turn-off signal to the second auxiliary switch when the charged voltage of the auxiliary-switch snubber capacitor is approximately equal to zero after the first main switch is turned on, so as to achieve soft-switching of the first and second auxiliary switches.
In another aspect of the present invention, the power conversion apparatus comprises a second auxiliary resonant circuit including serial-connected third and fourth auxiliary switches and a resonant inductor connected in series with the fourth auxiliary switch, wherein the second auxiliary resonant circuit is connected with each of the negative side of the DC power supply and the junction between the pair of main switches. The power conversion apparatus further includes a diode connected to each of the third and fourth auxiliary switches so as to become reverse biased with respect to the DC power supply, and voltage detecting means for detecting the voltage across each of the main switches and auxiliary switches. In this case, the control circuit is applied with a voltage signal as an input representing the voltage across each of the main switches and auxiliary switches from the voltage detecting means. The control circuit provides a turn-on signal to the third and fourth auxiliary switches according to the input, before a turn-on signal as the switching signal is provided to the second main switch, when the first main switch is in ON-state to allow the load current to pass through the first main switch and the load current is less than a threshold associated with the product of multiplying the capacity of the main-switch snubber capacitor by the power supply voltage of the DC power supply, so as to turn on the third and fourth auxiliary switches to direct the current from the DC power supply to the resonant inductor. Further, the control circuit provides a turn-off signal to the first main switch when the current of the resonant inductor goes up approximately to the threshold, so as to turn off the first main switch. The control circuit may be adapted to provide the turn-on signal to the second main switch, when the third and fourth auxiliary switches are in ON-state and the current passing through the resonant inductor is refluxed from the third and fourth auxiliary switches through the diode connected in parallel with the second main switch.
The control circuit according another aspect of the present invention may be adapted to provide the turn-off signal to the third auxiliary switch after the second main switch is turned on, when the initial voltage of the auxiliary-switch snubber capacitor is approximately equal to the voltage of the DC power supply, and to provide the turn-off signal to the fourth auxiliary switch after the second main switch is turned on, when the initial voltage of the auxiliary-switch snubber capacitor is approximately equal to zero, so as to achieve soft-switching of the third and fourth auxiliary switches. The control circuit may also be adapted to provide the turn-off signal to the first main switch without providing any turn-on signal to the third and fourth auxiliary switches, when the load current is larger than the threshold, so as to achieve soft-switching of the first main switch. The aforementioned threshold may be defined by the following formula;
Ith=Crxc3x97Vin/tmax 
where Ith is the threshold, Cr being the capacity of the main-switch snubber capacitor connected in parallel with the main switch, Vin being the voltage of the DC power supply, and tmax being the maximum allowable value of the time required for the load current to commutate from one of the first and second main switches to the other main switch.
The control circuit according to another aspect of the present invention provides a turn-on signal to the third and fourth auxiliary switches according to a voltage signal as an input representing the voltage across each of the main switches and the auxiliary switches from the voltage detecting means, before a turn-on signal as the switching signal is provided to the second main switch, and the control circuit also provides the turn-on signal to the third and fourth auxiliary switches when the load current passes through the diode connected in parallel with the first main switch, so as to turn on the third and fourth auxiliary switches to direct the current from the DC power supply to the resonant inductor. Then, a resonant circuit is formed by the resonant inductor and the snubber capacitors connected in parallel with the main switches when the current of the resonant inductor goes up approximately to the load current, and the control circuit outputs a signal for turning on the second main switch when the voltage across the second main switch goes down approximately to zero through the resonance in the resonance circuit.
According to another aspect of the present invention, the control circuit may be adapted to provide a turn-off signal to the third auxiliary switch when the charged voltage of the auxiliary-switch snubber capacitor is approximately equal to the voltage of the DC power supply after the second main switch is turned on, and to provide the turn-off signal to the fourth auxiliary switch when the charged voltage of the auxiliary-switch snubber capacitor is approximately equal to zero after the second main switch is turned on, so as to achieve soft-switching of the third and fourth auxiliary switches. In this case, the control circuit may also be adapted to provide a turn-on signal to the first and second auxiliary switches, before a turn-on signal as the switching signal is provided to the first main switch, when the second main switch is in ON-state to allow the load current to pass through the second main switch and the load current is less than a threshold associated with the product of multiplying the capacity of the main-switch snubber capacitor by the power supply voltage of the DC power supply, so as to turn on the first and second auxiliary switches to direct the current from the DC power supply to the resonant inductor, and then to provide a turn-off signal to the second main switch when the current of the resonant inductor goes up approximately to the threshold, so as to turn off the second main switch. Further, the control circuit may be adapted to provide the turn-on signal to the first main switch, when the first and second auxiliary switches are in ON-state and the current passing through the resonant inductor is refluxed from the first and second auxiliary switches through the diode connected in parallel with the first main switch.
In another aspect, a power conversion apparatus according to the present invention comprises a second auxiliary resonant circuit including serial-connected third and fourth auxiliary switches and a resonant inductor connected in series with the fourth auxiliary switch, wherein the second auxiliary resonant circuit is connected with each of the negative side of the DC power supply and the junction between the pair of main switches. The power conversion apparatus further includes a diode connected to each of the third and fourth auxiliary switches so as to become reverse biased with respect to the DC power supply, and voltage detecting means for detecting the voltage across each of the main switches and auxiliary switches. In this case, the control circuit is applied with a voltage signal as an input representing the voltage across each of the main switches and auxiliary switches from the voltage detecting means, and the control circuit provides a turn-on signal to the third and fourth auxiliary switches according to the input before a turn-on signal as the switching signal is provided to the second main switch. The control circuit also provides the turn-on signal to the third and fourth auxiliary switches when the load current passes through the diode connected in parallel with the second main switch, so as to turn on the third and fourth auxiliary switches to direct the current from the DC power supply to the resonant inductor. Then, a resonant circuit is formed by the resonant inductor and the snubber capacitors connected in parallel with the main switches when the current of the resonant inductor goes up approximately to the load current, and the control circuit outputs a signal for turning on the second main switch when the voltage across the second main switch goes down approximately to zero through the resonance in the resonance circuit.
In another aspect of the present invention, the power conversion apparatus may further includes serial-connected first and second auxiliary switches which are connected between the negative side of the DC power supply and the inductor so as to form a first auxiliary resonant circuit, an auxiliary-switch snubber capacitor connected between the junction between the first and second auxiliary switches and the junction between the third and fourth auxiliary switches, and a diode connected to each of the first and second auxiliary switches so as to become reverse biased with respect to the DC power supply. In this case, the control circuit provides a turn-off signal to the third auxiliary switch when the charged voltage of the auxiliary-switch snubber capacitor is approximately equal to the voltage of the DC power supply after the second main switch is turned on, and to provide the turn-off signal to the fourth auxiliary switch when the charged voltage of the auxiliary-switch snubber capacitor is approximately equal to zero after the second main switch is turned on, so as to achieve soft-switching of the third and fourth auxiliary switches.
In another aspect of the present invention, a power conversion apparatus comprises a first auxiliary resonant circuit including serial-connected first and second auxiliary switches and a resonant inductor connected in series with the second auxiliary switch, wherein the first auxiliary resonant circuit is connected with each of the positive side of the DC power supply and the junction between the pair of main switches. The power conversion apparatus further includes a diode connected to each of the first and second auxiliary switches so as to become reverse biased with respect to the DC power supply, and voltage detecting means for detecting the voltage across each of the main switches and auxiliary switches. In this case, the control circuit is applied with a voltage signal as an input representing the voltage across each of the main switches and auxiliary switches from the voltage detecting means, and the control circuit then provides a turn-on signal to the first and second auxiliary switches according to the input, before a turn-on signal as the switching signal is provided to the first main switch, when the second main switch is in ON-state to allow the load current to pass through the second main switch and the load current is less than a threshold associated with the product of multiplying the capacity of the main-switch snubber capacitor by the power supply voltage of the DC power supply, so as to turn on the first and second auxiliary switches to direct the current from the DC power supply to the resonant inductor. Further, the control circuit provides a turn-off signal to the second main switch when the current of the resonant inductor goes up approximately to the threshold, so as to turn off the second main switch. The control circuit may be adapted to provide the turn-on signal to the first main switch, when the first and second auxiliary switches are in ON-state and the current passing through the resonant inductor is refluxed from the first and second auxiliary switches through the diode connected in parallel with the first main switch.
According to another aspect of the present invention, the control circuit may be adapted to provide the turn-off signal to the first auxiliary switch after the first main switch is turned on, when the initial voltage of the auxiliary-switch snubber capacitor is approximately equal to the voltage of the DC power supply, and to provide the turn-off signal to the second auxiliary switch after the first main switch is turned on, when the initial voltage of the auxiliary-switch snubber capacitor is approximately equal to zero, so as to achieve soft-switching of the first and second auxiliary switches. The control circuit can also achieve soft-switching in the second main switch by providing the turn-off signal to the second main switch without providing any turn-on signal to the first and second auxiliary switches, when the load current is larger than a threshold. The threshold in this case may be defined by the same formula as described above.
According to another aspect of the present invention, the control circuit is applied with a voltage signal as an input representing the voltage across each of the main switches and auxiliary switches from the voltage detecting means. Then, the control circuit provides a turn-on signal to the first and second auxiliary switches according to the input before a turn-on signal as the switching signal is provided to the first main switch, and provides the turn-on signal to the first and second auxiliary switches when the load current passes through the diode connected in parallel with the second main switch, so as to turn on the first and second auxiliary switches to direct the current from the DC power supply to the resonant inductor. Then, the control circuit outputs a signal for turning on the first main switch when the voltage across the first main switch goes down approximately to zero through the resonance in a resonance circuit formed by the resonant inductor and the snubber capacitors connected in parallel with the main switches when the current of the resonant inductor goes up approximately to the load current.
According another aspect of the present invention, the control circuit may be adapted to provide a turn-off signal to the first auxiliary switch when the charged voltage of the auxiliary-switch snubber capacitor is approximately equal to the voltage of the DC power supply after the first main switch is turned on, and to provide the turn-off signal to the second auxiliary switch when the charged voltage of the auxiliary-switch snubber capacitor is approximately equal to zero after the second main switch is turned on, so as to achieve soft-switching of the first and second auxiliary switches. In this case, the control circuit may also be adapted to provide a turn-on signal to the third and fourth auxiliary switches, before a turn-on signal as the switching signal is provided to the second main switch, when the first main switch is in ON-state to allow the load current to pass through the first main switch and the load current is less than a threshold associated with the product of multiplying the capacity of the main-switch snubber capacitor by the power supply voltage of the DC power supply, so as to turn on the third and fourth auxiliary switches to direct the current from the DC power supply to the resonant inductor, and then to provide a turn-off signal to the first main switch when the current of the resonant inductor goes up approximately to the threshold, so as to turn off the first main switch. Further, the control circuit may be adapted to provide the turn-on signal to the second main switch, when the third and fourth auxiliary switches are in ON-state and the current passing through the resonant inductor is refluxed from the third and fourth auxiliary switches through the diode connected in parallel with the second main switch.
In another aspect of the present invention, the control circuit provides a turn-on signal to the third and fourth auxiliary switches and provide a turn-off signal to the first main switch, before a turn-on signal as the switching signal is provided to the second main switch, when the first main switch is in ON-state to allow the load current to pass through the first main switch and the load current is less than a threshold associated with the product of multiplying the capacity of the main-switch snubber capacitor by the power supply voltage of the DC power supply, so as to turn on the third and fourth auxiliary switches to generate a resonance between the resonant inductor and the snubber capacitor, to achieve the commutation between the main switches. Similarly, in light load, the control circuit provides a turn-on signal to the third and fourth auxiliary switches and provide a turn-off signal to the second main switch, before a turn-on signal as the switching signal is provided to the first main switch, when the second main switch is in ON-state to allow the load current to pass through the second main switch and the load current passes through the second main switch the control circuit provides a turn-on signal to the third and fourth auxiliary switches and provide a turn-off signal to the first main switch, before a turn-on signal as the switching signal is provided to the second main switch, when the first main switch is in ON-state to allow the load current to pass through the first main switch and the load current is less than a threshold associated with the product of multiplying the capacity of the main-switch snubber capacitor by the power supply voltage of the DC power supply, so as to turn on the third and fourth auxiliary switches to generate a resonance between the resonant inductor and the snubber capacitor, to achieve the commutation between the main switches.
The control according to the above aspect can advantageously eliminate the need for detecting current and thereby facilitate simplifying the control. Further, since no indicator current is passed through the main switches, the turn-off loss in the mains switches is not increased.